Durable glass ceramic cover glass for electronic devices

ABSTRACT

The invention relates to glass articles suitable for use as electronic device housing/cover glass which comprise a glass ceramic material. Particularly, a cover glass comprising an ion-exchanged glass ceramic exhibiting the following attributes (1) optical transparency, as defined by greater than 90% transmission at 400-750 nm; (2) a fracture toughness of greater than 0.6 MPa·m1/2; (3) a 4-point bend strength of greater than 350 MPa; (4) a Vickers hardness of at least 450 kgf/mm2 and a Vickers median/radial crack initiation threshold of at least 5 kgf; (5) a Young&#39;s Modulus ranging between about 50 to 100 GPa; (6) a thermal conductivity of less than 2.0 W/m° C., and (7) and at least one of the following attributes: (i) a compressive surface layer having a depth of layer (DOL) greater and a compressive stress greater than 400 MPa, or, (ii) a central tension of more than 20 MPa.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is an application for reissue of U.S. Ser. No. 14/074,803, filedNov. 8, 2013, issued as U.S. Pat. No. 9,604,871 B2, and is acontinuation of U.S. Ser. No. 16/045,438 filed Jul. 25, 2018, which isalso an application for reissue of U.S. Pat. No. 9,604,871 B2. U.S. Ser.No. 14/074,803, filed Nov. 8, 2013, issued as U.S. Pat. No. 9,604,871 B2claims the benefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 61/724,039, filed on Nov. 8, 2012.

, the content of whichEach application and patent identified in theprevious sentence is relied upon and incorporated herein by reference inits entirety.

Notice: More than one reissue application has been filed for the reissueof U.S. Ser. No. 14/074,803, filed Nov. 8, 2013, issued as U.S. Pat. No.9,604,871 B2. The reissue applications are U.S. Ser. No. 16/045,438filed Jul. 25, 2018 and the present application.

TECHNICAL FIELD

The invention is directed to glass ceramic materials that can be used ascover glasses for electronic devices. In particular, the invention isdirected to essentially colorless glass ceramics having high opticaltransmission in the visible region winch are suitable for use as coverglass materials for electronic devices.

BACKGROUND

In the past decade portable electronic devices such as laptops, PDAs,media players, cellular phones, etc. (frequently referred to as“portable computing devices”), have become small, light and powerful.One factor contributing to the development and availability of thesesmall devices is the manufacturer's ability to reduce of the device'selectronic components to ever smaller and smaller sizes whilesimultaneously increasing both the power and or operating speed of suchcomponents. However, the trend to devices that are smaller, lighter andmore powerful presents a continuing challenge regarding design of somecomponents of the portable computing, devices.

One particular challenge is associated with the design of the portablecomputing devices is the cover glass used as both the display screen andfor protection of the sensitive optical components that providesvisualization means for the devices. This design challenge generallyarises from two conflicting design goals—the desirability of making thedevice lighter and thinner while continuing to make the display as largeas possible, and the desirability of making the cover glass stronger andmore scratch and fracture resistant. Thinner cover glasses tend to bemore flexible, are more prone to breaking than thinker cover glasses.Unfortunately, the increased weight of the stronger, thicker glasses maylead to user dissatisfaction.

In view of the foregoing problems with existing cover glass materials,there is a need for improved cover glasses for portable computingdevices. In particular, there is a need for cover glass materials thatare more cost effective, lighter, and stronger than current designs.

SUMMARY

In one embodiment the invention relates to portable electronic devicescapable of wireless communications having glass ceramic cover glasses.

A first aspect comprises an article suitable as a cover glass for aportable electronic device, the article comprising a glass ceramicexhibiting: a. optical transparency of greater than 60%, as defined bythe transmission of light over the range of from 400-750 nm through 1 mmof the glass ceramic; b. colorlessness, as defined by having the valuesof L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4 on the CIE 1976 Lab color spaceas measured in transmission through 1 mm of glass ceramic; c. at leastone of the following attributes: (i) a fracture toughness of greaterthan 0.60 MPa·m^(1/2); (ii) a 4-point bend strength of greater than 350MPa; (iii) a Vickers hardness of at least 450 kgf/mm²; (iv) a Vickersmedian/radial crack initiation threshold of at least 5 kgf; (v) aYoung's Modulus ranging between 50 to 100 GPa; and (vi) a thermalconductivity of less than 2.0 W/m° C.; and d. at least one of thefollowing attributes: (i) a compressive surface layer having a depth oflayer (DOL) greater than or equal to 20 μm and a compressive stressgreater than 400 MPa, or, (ii) a central tension of more than 20 MPa.

In some embodiments, the glass ceramic is ion exchanged. In someembodiments, the glass ceramic exhibits optical transparency of greaterthan 80%, as defined by the transmission, of light over the range offrom 400-750 nm through 1 mm of the glass ceramic. In some embodiments,the glass ceramic exhibits colorlessness, as defined by having colorspace coordinates of L*≥90; 0.08≥a*≥−0.08; and 0.3≥b*≥−0.3 on the CIE1976 Lab color space as measured in transmission through 1 mm of glassceramic. In some embodiments, the glass ceramic exhibits a Young'sModulus ranging between 50 and 75 GPa. In some embodiments, the glassceramic exhibits an 4-point bend strength of greater than 475 MPa. Insome embodiments, the glass ceramic exhibits a Vickers hardness of atleast 500 kgf/mm² and Vickers median/radial crack initiation thresholdof greater than 10 kgf. In some embodiments, the glass ceramic exhibitsa thermal conductivity of less than 1.5 W/m° C. In some embodiments, theglass ceramic article is transparent and exhibits at least one surfacehaving a Ra roughness of less than less than 50 nm. In some embodiments,the glass ceramic exhibits a near-infra-red transparency of greater than80% at a wavelength ranging from 750 to 2000 nm. In some embodiments,the glass ceramic exhibits an overall thickness of 1.2 mm andcompressive layer exhibiting a DOL of ranging between 40 to 80 μm andthe compressive layer exhibits a compressive stress of 525 MPa. In someembodiments, the glass ceramic comprises a b-quartz, high aluminab-quartz, transparent b-spodumene, transparent spinel, transparentmullite. In some embodiments, the glass ceramic is fusion formable.

Another aspect comprises an article suitable as a cover glass for aportable electronic device, the article comprising a glass ceramicexhibiting: a. optical transparency of greater than 60%, as defined bythe transmission of light over the range of from 400-750 nm through 1 mmof the glass ceramic; b. colorlessness, as defined by having the valuesof L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4 on the CIE 1976 Lab color spaceas measured in transmission through 1 mm of glass ceramic; and c. atleast one of the following attributes: (i) a fracture toughness ofgreater than 1.0 MPa·m^(1/2); (ii) MOR of greater than 135 MPa; (iii) aKnoop hardness of at least 400 kg/mm²; (iv) a thermal conductivity ofless than 4 W/m° C.; and (v) a porosity of less than 0.1%.

In some embodiments, the glass ceramic is ion exchanged. In someembodiments, the glass ceramic exhibits optical transparency of greaterthan 80%, as defined by the transmission of light over the range of from400-750 nm through 1 mm of the glass ceramic. In some embodiments, theglass ceramic exhibits colorlessness, as defined by having color spacecoordinates of L*≥90; 0.08≥a*≥−0.08; and 0.3≥b*≥−0.3 on the CIE 1976 Labcolor space as measured in transmission through 1 mm of glass ceramic.In some embodiments, the glass ceramic exhibits a Young's Modulusranging between 50 and 75 GPa. In some embodiments, the glass ceramicexhibits an 4-point bend strength of greater than 475 MPa. In someembodiments, the glass ceramic exhibits a Vickers hardness of at least500 kgf/mm² and Vickers median/radial crack initiation threshold ofgreater than 10 kgf. In some embodiments, the glass ceramic exhibits athermal conductivity of less than 1.5 W/m° C. In some embodiments, theglass ceramic article is transparent and exhibits at least one surfacehaving a Ra roughness of less than less than 50 nm. In some embodiments,the glass ceramic exhibits a near-infra-red transparency of greater than80% at a wavelength ranging from 750 to 2000 nm. In some embodiments,the glass ceramic exhibits an overall thickness of 1.2 mm andcompressive layer exhibiting a DOE of ranging between 40 to 80 μm andthe compressive layer exhibits a compressive stress of 525 MPa. In someembodiments, the glass ceramic comprises a β-quartz, high aluminaβ-quartz, transparent β-spodumene, transparent spinel, transparentmullite. In sonic embodiments, the glass ceramic is fusion formable.

In certain embodiments the thickness of glass article housing/enclosureor cover is less than 2 mm and exhibits an aspect ratio greater than 25to 1 (i.e., maximum dimension of length, width, or diameter which is >25times greater than the thickness).

The ion exchanged glass article can be used in a variety of consumerelectronic articles, for example, cellphones and other electronicdevices capable of wireless communication, music players, notebookcomputers, PDA's, game controllers, computer “mice”, electronic bookreaders and other devices.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details may beset forth in order to provide a thorough understanding of embodiments ofthe invention. However, it will be clear to one skilled in the art whenembodiments of the invention may be practiced without some or all ofthese specific details. In other instances, well-known features orprocesses may not be described in detail so as not to unnecessarilyobscure the invention. In addition, like or identical reference numeralsmay be used to identify common or similar elements. Moreover, unlessotherwise defined, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. In case of conflict, the presentspecification, including the definitions herein, will control.

Although other methods and can be used in the practice or testing of theinvention, certain suitable methods and materials are described herein.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are embodiments of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein.

Thus, if a class of substituents A, B, and C are disclosed as well as aclass of substituents D, E, and F, and an example of a combinationembodiment, A-D is disclosed, then each is individually and collectivelycontemplated. Thus, in this example, each of the combinations A-E, A-F,B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated andshould be considered disclosed from disclosure of A, B, and/or C; D, E,and/or F; and the example combination A-D. Likewise, any subset orcombination of these is also specifically contemplated and disclosed.Thus, for example, the subgroup of A-E, B-F, and C-E are specificallycontemplated and should be considered disclosed from disclosure of A, B,and/or C; D, E, and/or F; and the example combination A-D. This conceptapplies to all aspects of this disclosure including, but not limited toany components of the compositions and steps in methods of making andusing the disclosed compositions. Thus, if there are a variety ofadditional steps that can be performed it is understood that each ofthese additional steps can be performed with any specific embodiment orcombination of embodiments of the disclosed methods, and that each suchcombination is specifically contemplated and should be considereddisclosed.

Moreover, where a range of numerical values is recited herein,comprising upper and lower values, unless otherwise stated in specificcircumstances, the range is intended to include the endpoints thereof,and all integers and fractions within the range. It is not intended thatthe scope of the invention be limited to the specific values recitedwhen defining a range. Further, when an amount, concentration, or othervalue or parameter is given as a range, one or more preferred ranges ora list of upper preferable values and lower preferable values, this isto be understood as specifically disclosing all ranges formed from anypair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether such pairs areseparately disclosed. Finally, when the term “about” is used indescribing a value or an end-point of a range, the disclosure should beunderstood to include the specific value or end-point referred to.

As used herein, the term “about” means that amounts, sizes,formulations, parameters, and other quantities and characteristics arenot and need not be exact, but may be approximate and/or larger orsmaller, as desired, reflecting tolerances, conversion factors, roundingoff, measurement error and the like, and other factors known to those ofskill in the art. In general, an amount, size, formulation, parameter orother quantity or characteristic is “about” or “approximate” whether ornot expressly stated to be such.

The term “or”, as used herein, is inclusive; more specifically, thephrase “A or B” means “A, B, or both A and B”. Exclusive “or” isdesignated herein by terms such as “either A or B” and “one of A or B”,for example.

The indefinite articles “a” and “an” are employed to describe elementsand components of the invention. The use of these articles means thatone or at least one of these elements or components is present. Althoughthese articles are conventionally employed to signify that the modifiednoun is a singular noun, as used herein the articles “a” and “an” alsoinclude the plural, unless otherwise stated in specific instances.Similarly, the definite article “the”, as used herein, also signifiesthat the modified noun may be singular or plural, again unless otherwisestated in specific instances.

For the purposes of describing the embodiments, it is noted thatreference herein to a variable being a “function” of a parameter oranother variable is not intended to denote that the variable isexclusively a function of the listed parameter or variable. Rather,reference herein to a variable that is a “function” of a listedparameter is intended to be open ended such that the variable may be afunction of a single parameter or a plurality of parameters.

It is noted that terms like “preferably,” “commonly,” and “typically,”when utilized herein, are not utilized to limit the scope of the claimedinvention or to imply that certain features are critical, essential, oreven important to the structure or function of the claimed invention.Rather, these terms are merely intended to identify particular aspectsof an embodiment of the present disclosure or to emphasize alternativeor additional features that may or may not be utilized in a particularembodiment of the present disclosure.

For the purposes of describing and defining the claimed invention it isnoted that the terms “substantially” and “approximately” are utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. The terms “substantially” and “approximately” are alsoutilized herein to represent the degree by which a quantitativerepresentation may vary from a stated reference without resulting in achange in the basic function of the subject matter at issue.

It is noted that one or more of the claims may utilize the term“wherein” as a transitional phrase. For the purposes of defining thepresent invention, it is noted that this term is introduced in theclaims as an open-ended transitional phrase that is used to introduce arecitation of a series of characteristics of the structure and should beinterpreted in like manner as the more commonly used open-ended preambleterm “comprising.”

As a result of the raw materials and/or equipment used to produce theglass composition of the present invention, certain impurities orcomponents that are not intentionally added, can be present in the finalglass composition. Such materials are present in the glass compositionin minor amounts and are referred to herein as “tramp materials.”

As used herein, a glass composition having 0 wt % of a compound isdefined as meaning that the compound, molecule, or element was notpurposefully added to the composition, but the composition may stillcomprise the compound, typically in tramp or trace amounts. Similarly,“sodium-free,” “alkali-free,” “potassium-free” or the like are definedto mean that the compound, molecule, or element was not purposefullyadded to the composition, but the composition may still comprise sodium,alkali, or potassium, but in approximately tramp or trace amounts.

As used herein the terms “display,” “display screen” and “cover glass”are used interchangeably to describe the sheet-like material that coversthe face of the electronic display on an electronic device. Use of“glass” in the term “cover glass” is meant to be inclusive of glassceramics.

As used herein, “optical transparency” or “optically transparent” meansthat the material has the ability of allowing light in the visibleregion to pass through the material without being scattered. In someembodiments, optically transparent means that greater than 40, 45, 50,55, 60, 65, 70, 75, 80, 85, 90, 95, 98, or 99% of the light in thevisible range is passed through 1 mm thickness of the material withoutbeing scattered. In some embodiments, the “visible region” comprisesfrom about 350 to about 800 nm. In some embodiments, the visible regioncomprises from about 390 to about 750 nm. In some embodiments, the lowerlimit of the visible region is about 350, 360, 370, 380, 390, 400, 410,420, 430, 440, 450, or 460 nm. In some embodiments, the upper limit ofthe visible region is about 850, 840, 830, 820, 810, 800, 790, 780, 770,760, 750, 740, 730, 720, 710, or 700 nm.

As used herein, “colorless” or “essentially colorless” means that thematerial has values of L*≥90; 0.1≤a*≤−0.1; and 0.4≤b*≤−0.4 on the CIE1976 Lab color space as measured in transmission through 1 mm of glassceramic. In some embodiments, L* is ≥85, ≥90, ≥91, ≥92, ≥93, ≥94, ≅95,≥96, ≥97, ≥98, or ≥90. In some embodiments, 0.15≥a*≥−0.15, 0.1≥a*≥−0.1,0.09≥a*≥−0.09, 0.08≥a*≥−0.08, 0.07≥a*≥−0.07, 0.06≥a*≥−0.06,0.051≥a*≥−0.05, 0.04≥a*≥−0.04, 0.03≥a*≥−0.03, or 0.02≥a*≥−0.02. In someembodiments, 0.45≥b*≥−0.45, 0.4≥b*≥−0.4, 0.35≥b*≥−0.35, 0.3≥b*≥−0.3,0.25≥b*≥−0.25, 0.2≥b*≥−0.2, 0.15≥b*≥−0.15, 0.1≥b*≥−0.1, 0.08≥b*≥−0.08,0.05≥b*≥−0.05, or 0.03≥b*≥−0.03. Colorless, as used herein, may includeboth intrinsically essentially colorless (because of the absence of ionsor ion pairs, i.e. the presence of ions or ion pairs which when exposedto visible light can undergo electronic transitions) and essentiallycolorless due to compensating coloration by development of acomplementary color in the material (see, e.g., U.S. Pat. No. 4,093,468,herein incorporated by reference in its entirety).

Regarding “colorless” or “essentially colorless” glass ceramics, thoughit appears the presence in a glass-ceramic of compounds impartingcoloration could be avoided by avoiding or minimizing the introductionof said compounds or precursors thereof into the raw materials, thesituation becomes more complex when certain necessary components caninteract with a color-imparting species in the glass-ceramic material.For example, it is known that the presence of Fe₂O₃ alone (no TiO₂) upto contents of the order of 300 ppm in a glass-ceramic is generally nota concern as regards coloration. However, the joint presence of Fe₂O₃and TiO₂ generates a characteristic yellowish tint. A number ofcommercial products which are otherwise known for their hightransparency retain this yellowish tint, in particular those sold by theApplicant under the trade name KERALITE® (described in European patentapplication EP 0 437 228), those sold by Schott AG under the trade nameROBAX® and those sold by Nippon Electric Glass under the trade nameNEOCERAM® N—O, because of the joint presence in their compositions ofTiO₂ and Fe₂O₃. Treating the raw materials used to reduce the Fe₂O₃content to below 150 ppm in particular is an expensive operation (anoption mentioned in Japanese patent application JP 2001-348250) and itis seen above that TiO₂ is the best performing nucleating agent,allowing ceramming to occur on reasonable time scales. To overcome thetechnical problem mentioned above—obtaining transparent β-quartzglass-ceramic materials with no yellowish coloration˜one possibleapproach seems to be to dispense with the presence of TiO₂ duringmanufacture.

“Glass ceramic” as used herein, describes any polycrystalline materialproduced through controlled crystallization of base glass. In someembodiments, glass-ceramics comprise from 30% to 90% [mol/mol]crystallinity. In some embodiments, the lower limit is about 20, 25, 30,35, 40, 45, or 50% [mol/mol] crystallinity. In some embodiments, theupper limit is about 95, 90, 85, 80, 75, 70, or 65% [mol/mol]crystallinity-

As is described herein below, the needs of the industry for more costeffective, smaller, lighter, and stronger cover glass materials may bemet by the use of durable glass ceramic compositions and articles as thecover glass for consumer electronics. Examples of devices which mayutilize the improved glass ceramic cover glass materials include, butare not limited to, cell phones, electronic tablets, music players,notebook computers, game controllers, computer “mice”, electronic bookreaders and other devices. These glass ceramic materials possess certainadvantages such as weight and/or resistance to impact damage (e.g.,denting) and scratching over the present materials, such as plastic andglass. Furthermore, the glass ceramics described herein are not onlydurable, but, unlike many of the materials presently used forhousings/enclosures/covers, in particular metallic housing/enclosures,the use of glass ceramics does not interfere with wirelesscommunications.

The glass ceramic material which is suitable for use in the cover glassof a portable electronic device may be formed from a variety ofglass-ceramic materials. In particular, numerous glass-ceramiccompositional families can be employed for this application. Glassceramics based on b-quartz, high alumina b-quartz, transparentb-spodumene, transparent spinel, transparent mullite, exist and may beused in embodiments. Specific examples of glass ceramics that may beused in the present disclosure include, but are not limited to, thosedescribed in U.S. Pat. Nos. 3,681,102, 4,519,828, 7,730,531, 4,940,674,4,341,543, 4,059,428, 4,526,873, 4,455,160, 7,507,681, 5,070,045,6,103,338, 5,968,219, 6,197,429, 5,968,857, 5,786,286, 8,143,179,6,844,278, 5,127,93, 6,248,678, 6,531,420, 6,632,757, 6,632,758,6,660,669, 7,300,896, 7,910,507, 7,105,232, 7,361,405, 7,763,832, U.S.application Ser. No. 13/212,587, U.S. Publ. Nos. 2012/0114955,2011/0092353, 2008/0199622, 2007/0270299, and PCT 2008/081728, all ofwhich are incorporated by reference in their entireties. Additionalglass ceramics for use in cover glass applications may be found in L. R.Pinckney, “Glass-Ceramics”, Kirk-Othmer Encyclopedia of ChemicalTechnology, 4th edition, Vol. 12, John Wiley and Sons, 627-644, 1994,herein incorporated by reference in its entirety.

Examples of glass ceramics embodied herein may be found in Table 1 (fromU.S. Pat. No. 3,681,102):

TABLE I 1 2 3 4 5 6 7 Percent: SiO₂ 64.8 64.8 66.0 65.2 65.1 66.0 67.8Al₂O₃ 18.5 15.7 16.0 17.2 17.2 16.0 17.0 ZnO 4.6 12.0 12.3 6.1 6.0 12.25.7 MgO 4.6 — — 4.7 4.7 — 3.8 ZrO₂ 7.5 7.5 5.7 6.5 6.5 5.6 5.6 Cr₂O₃ — —— 0.3 0.5 0.2 0.1 Melting temp., ° C. 1,650 1,800 1,650 1,650 1,6501,650 1,650

Table 2 (U.S. Pat. No. 4,519,828):

TABLE 2 1 2 3 4 5 6 7 8 SiO₂ 45 40 45 45 60 45 50 40 B₂O₃ 15 15 20 15 1530 15 25 Al₂O₃ 30 35 25 30 20 20 25 25 K₂O 10 10 10 — 5 5 10 10 Na₂O — —— 10 — — — — Cr₂O₃ 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 As₂O₅ 0.050.05 0.05 0.05 0.05 0.05 0.05 0.05 Al₂O₃:RO + 2.86 3.22 2.30 1.82 3.693.69 3.69 2.30 R₂O 9 10 11 12 13 14 15 16 SiO₂ 20 49 65 60 58 40 42.542.5 B₂O₃ 25 15 15 20 20 35 30 30 Al₂O₃ 35 30 15 15 20 20 20 20 K₂O 20 —5 5 — 5 7.5 7.5 CaO — 6 — — — — — — MgO — — — — 2 — — — Cr₂O₃ 0.05 0.050.05 0.05 0.05 0.05 0.10 0.30 As₂O₅ 0.05 0.05 0.05 0.05 0.05 0.05 0.050.05 Al₂O₃:RO + 1.62 2.74 2.76 2.76 3.95 3.69 2.46 2.46 R₂O 17 18 19 2021 22 23 24 SiO₂ 53.5 55 45 45 50 15 55 55 B₂O₃ 25 25 20 20 25 30 20 20Al₂O₃ 20 20 25 25 20 25 20 20 K₂O — — — — — — 5 5 Li₂O 1.5 — — — — — — —BaO — — 10 — — — — — SrO — — — 10 5 — — — PbO — — — — — 30 — — Cr₂O₃0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.20 As₂O₅ 0.05 0.05 0.05 0.05 0.050.05 0.05 0.05 Al₂O₃:RO + 3.93 ∞ 3.76 2.53 4.06 1.83 3.69 3.69 R₂O 25 2627 28 29 30 31 32 SiO₂ 45 45 35 60 55 45 55 50 B₂O₃ 20 20 20 17.5 15 2515 20 Al₂O₃ 25 25 20 20 25 20 20 20 K₂O 10 10 — — — — — — Na₂O — — — 2.5— — — — Cs₂O — — 25 — — — — — MgO — — — — 5 — — — BaO — — — — — 10 10 —SrO — — — — — — — 10 Cr₂O₃ 0.10 0.20 0.20 0.05 0.05 0.05 0.05 0.05 As₂O₅0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Al₂O₃:RO + 2.30 2.30 2.21 3.041.98 3.01 3.01 2.03 R₂O 33 34 35 36 37 38 39 40 SiO₂ 57.5 40 60 60 59 5965 65 B₂O₃ 20 22.5 20 25 6 20 17.5 10 Al₂O₃ 20 22.5 20 15 25 20 15 20K₂O 2.5 — — — 10 — — — PbO — 15 — — — — — — Na₂O — — — — — 1 2.5 5 Cr₂O₃0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 As₂O₅ 0.05 0.5 0.5 0.5 0.5 0.50.5 0.5 Al₂O₃:RO + 7.37 3.82 ∞ ∞ 2.30 12.2 3.63 2.43 R₂O

Table 3 (U.S. Pat. No. 4,526,873):

TABLE 3 Oxide 1 2 3 4 5 6 7 8 SiO₂ 50 45 52.5 53.5 48 48 44 44 B₂O₃ 0 1517.5 15.0 24 20 17.5 17.5 Al₂O₃ 40 30 22.5 20.0 21 21 25.0 25.0 BaO 10 —— — — — — — K₂O 0 10 — 2.5 2 2 3.5 3.5 Na₂O — — 2.5 — — — — — MgO — — —— 3 — — — ZnO — — 5.0 9.0 2 9 10.0 10.0 Cr₂O₃ 0.05 0.05 0.10 0.20 0.10.1 0.10 0.05 As₂O_(3.5) — — — — — — — 0.2 Al₂O₃/ 6.0 2.86 2.16 1.431.71 1.56 1.53 1.53 RO + R₂O

Table 4 (U.S. Pat. No. 4,940,674):

TABLE 4 1 2 3 4 5 6 7 SiO₂ 65.6 65.5 65.5 65.5 65.4 65.4 65.4 Al₂O₃ 19.919.9 19.9 19.9 19.8 19.7 19.7 B₂O₃ 2.3 2.3 2.3 2.3 2.3 2.0 1.9 Li₂O 4.34.3 4.3 4.3 4.3 4.3 4.3 K₂O 1.9 1.9 1.9 1.9 1.9 1.8 1.8 BaO 1.3 1.3 1.31.3 1.3 1.9 2.0 TiO₂ 2.5 2.5 2.5 2.5 2.5 2.5 2.5 ZrO₂ 1.2 1.2 1.2 1.21.2 1.2 1.2 Cr₂O₃ — 0.007 0.015 0.025 0.014 0.006 0.008 Fe₂O₃ — — — —0.11 0.085 0.040 Co₃O₄ — — — — 0.013 0.006 0.003 As₂O₃ 1.0 1.0 1.0 1.01.0 1.0 1.0

Table 5 (U.S. Pat. No. 5,968,857):

TABLE 5 Glass-Ceramic Compositions and Properties Oxide 1 2 3 4 5 6 7SiO₂ 57.5 61.5 60.2 58.3 58.8 59.6 58.9 Al₂O₃ 22.5 18.4 18.4 20.2 20.418.4 20.4 ZnO 8.5 8.1 10.6 8.4 6.8 8.6 7.7 MgO 4.2 4.0 2.8 4.2 5.0 4.34.6 Cs₂O — — — — — — — BaO — — — — — — — TiO₂ 7.3 3.0 3.0 3.0 3.0 3.03.2 ZrO₂ — 5.0 5.0 5.0 5.0 5.0 5.3 NH₄NO₃ 1.0 1.0 1.0 — — — 1.0 As₂O₃0.5 0.5 0.5 — — — 0.5 CaO — — — — — — — ΣRO/Al₂O₃ 0.95 1.11 1.11 1.051.05 1.18 1.05 H.T. 800/1 800/1 800/1 800/1, 800/1, 800/1, 800/1, 900/2900/2 875/2 900/2 900/2 900/2 900/2 CTE(10⁻⁷/° C.) 39.4 35.3 34.9 37.737.1 35.1 37.5 Strain Point 946 937 935 946 948 930 940 Density 2.762.72 2.76 Liq. Temp. 1475 1495 1500 Liq. Visc. 350 T @ 300 poise 1490 T@ 10³ poise 1393 T @ 10⁴ poise 1257 T @ 10⁵ poise 1162 Oxide 8 9 10 1112 13 14 SiO₂ 58.9 57.0 55.7 57.7 55.0 53.6 56.5 Al₂O₃ 20.4 18.3 19.320.0 19.2 21.2 19.7 ZnO 7.7 8.4 9.0 8.5 13 11.3 9.7 MgO 4.6 2.0 2.0 3.5— 2.0 2.0 Cs₂O — 4.1 4.0 2.1 2.0 2.0 — BaO — 2.0 2.0 — 3.1 2.0 4.6 TiO₂5.3 5.1 5.0 5.1 4.9 5.0 5.0 ZrO₂ 3.2 3.1 3.0 3.1 2.9 3.0 2.5 NH₄NO₃ 1.0— — — — — 1.0 As₂O₃ 0.5 — — — — — 0.5 ΣRO/Al₂O₃ 1.05 1.0 1.0 1.0 1.0 1.01.0 H.T. 800/1 800/1 800/1 800/1 800/1 800/1 800/1 900/2 900/2 900/2900/2 900/2 900/2 900/2 CTE(10⁻⁷/° C.) 38.3 37.1 39.1 37.5 38.9 40.339.1 Strain Point 943 891 905 948 915 928 868 Density 2.75 Liq. Temp.1435 1475 1460 Liq. Visc. 700 T @ 300 poise 1550 1530 1500 1500 T @ 10³poise 1443 1420 1420 1400 T @ 10⁴ poise 1291 1280 1265 1260 T @ 10⁵poise 1180 1200 1190 1160 Oxide 15 16 17 18 19 20 SiO₂ 57.4 59.0 57.357.4 58.8 63.0 Al₂O₃ 18.5 19.1 18.5 17.7 20.0 17.8 ZnO 13.0 9.0 11.610.1 6.7 5.7 MgO — 2.5 2.4 — 4.9 4.2 Cs₂O — — — — — — BaO 3.1 2.1 2.03.2 1.6 1.3 TiO₂ 5.0 5.1 5.0 5.1 5.0 5.0 ZrO₂ 3.0 3.1 3.0 3.1 3.0 3.0NH₄NO₃ — — — — — — As₂O₃ — — — — 0.5 0.5 La₂O₃ — — — 3.4 — — ΣRO/Al₂O₃1.0 1.0 1.2 1.0 — — H.T. 800/1, 800/1, 800/1, 800/1, 800/1, 800/1, 900/2900/2 900/2 900/2 1000/2 1000/2 CTE(10⁻⁷/° C.) 37.1 37.6 38.4 37.3 36.833.7 Strain Point 933 913 922 909 899 908 Density 2.76 2.74 2.67 Liq.Temp/° C. 1425 1360 1460 Liq. Visc. 950 T @ 300 poise 1510 1520 14851530 T @ 10³ poise 1400 1416 1385 1425 T @ 10⁴ poise 1260 1275 1245 1280T @ 10⁵ poise 1160 1180 1140 1170

Table 6 (U.S. Pat. No. 6,248,678):

TABLE 6 wt % 1 2 3 4 5 6 7 SiO₂ 64.8 63.4 66.3 67.0 65.8 68.7 70.7 Al₂O₃15.8 17.3 15.5 15.7 15.4 14.1 12.9 ZnO 7.5 6.4 5.8 3.9 7.0 5.2 4.8 MgO2.1 3.2 2.8 3.9 2.2 2.6 2.4 BaO 1.7 1.7 1.6 1.6 1.6 1.4 1.3 TiO₂ 5.0 5.05.0 5.0 5.0 5.0 5.0 ZrO₂ 3.0 3.0 3.0 3.0 3.0 3.0 3.0 NH₄NO₃ 1.0 1.0 1.01.0 1.0 1.0 1.0 As₂O₅ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 H.T.  800/1,  800/1, 800/1,  800/1,  800/1,  800/1,  800/1, 1000/2 1000/2 1000/2 1000/21000/2 1000/2 1000/2 CTE(×10⁻⁷/° C.) 31.3 32.7 30.2 30.4 30.9 28.3 27.3Strain pt ° C. 910 903 924 898 907 902 908 Anneal pt ° C. 988 982 1000990 Density (g/cm³) 2.62 2.59 2.64 2.59 2.56 Density: glass 2.55 2.592.54 2.52 E-mod. (10⁶ psi) 12.8 12.9 12.6 Liq. Temp ° C. 1480 1450 14851485 1490 1490 1475 Approx. Visc. At Liq. 1800 1500 2500 Temp. T ° C. @10³ p 1532 1518 1572 T ° C. @ 10⁴ p 1363 1353 1395 T ° C. @ 10⁵ p 12421235 1270 T ° C. @ 10⁶ p 1149 1146 1176 wt % 8 9 10 11 12 13 14 SiO₂72.4 71.3 70.2 71.6 72.4 73.2 71.6 Al₂O₃ 11.9 13.0 12.8 12.7 11.9 13.012.7 ZnO 4.4 3.2 5.8 4.4 4.4 4.4 4.3 MgO 2.2 3.2 1.8 2.1 2.2 2.2 2.1 BaO1.2 1.3 1.3 1.2 1.2 1.2 — B₂O₃ — — — — 1.0 — — Cs₂O — — — — — — 2.2 TiO₂5.0 5.0 5.0 5.0 5.0 4.0 5.0 ZrO₂ 3.0 3.0 3.0 3.0 3.0 2.0 2.0 NH₄NO₃ 1.01.0 1.0 1.0 1.0 1.0 1.0 As₂O₅ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 H.T. 800/1,800/1, 800/1, 800/1, 800/1, 800/1, 800/1, 1000/2 1000/2 1000/2 1000/21000/2 1000/2 1000/2 CTE(×10⁻⁷/° C.) 25.2 27.3 26.7 25.2 26.3 24.7 25.3Strain pt ° C. 909 908 916 938 878 929 910 Anneal pt ° C. 991 988 9971014 1006 993 Density (g/cm³) 2.54 2.54 2.57 2.54 2.53 2.52 2.54Density: glass 2.50 2.50 2.53 2.50 2.47 2.49 Liq. Temp ° C. 1485 14801475 1480 1490 1440 Approx. Visc. At Liq. 6500 15,500 Temp. T ° C. @ 10³p T ° C. @ 10⁴ p 1449 1470 T ° C. @ 10⁵ p 1316 1327 T ° C. @ 10⁶ p 12221217

Table 7 (U.S. Pat. No. 6,531,420):

TABLE 7 1 2 3 4 5 6 7 8 SiO₂ 41.9 47.1 45.5 44.8 40.1 40.0 49.5 43.9Al₂O₃ 11.7 14.0 14.0 14.0 11.3 12.0 14.5 11.7 ZnO 34.0 19.0 24.1 26.631.8 36.0 26.6 MgO 4.5 10.3 3.1 Li₂O 1.7 7.0 3.5 1.7 1.7 2.5 7.6 4.0 K₂O10.7 12.9 12.9 12.9 10.6 9.5 13.3 10.7 Na₂O TiO₂ 4.8 2.7 ZrO₂ P₂O₅ °C.-hr 750-2 750-2 750-2 750-2 750-2 750-2 750-2 750-2 Glass-CeramicTransparency 4 2 2 3 4 4 2 2 17 18 19 20 21 22 23 24 SiO₂ 45.5 52.3 49.053.6 54.4 61.3 67.2 65.3 Al₂O₃ 14.0 14.4 14.3 13.9 14.1 12.8 13.0 13.9ZnO 23.5 17.2 16.8 13.9 9.1 9.7 MgO 12.4 10.2 Li₂O 4.0 9.2 7.5 9.0 9.07.9 7.3 7.7 K₂O 13.0 8.6 13.2 Na₂O 3.1 TiO₂ 2.8 ZrO₂ 2.8 P₂O₅ 6.4 5.74.1 3.4 3.4 ° C.-hr 650-1 725-2 725-1 650-2 650-2 650-2 650-2 650-2750-4 700-4 700-4 700-4 700-4 Glass-Ceramic Transparency 3 2 4 1 3 4 4 29 10 11 12 13 14 15 16 SiO₂ 51.6 48.2 49.0 50.0 43.0 46.3 42.1 53.4Al₂O₃ 14.1 16.0 16.0 16.0 11.7 14.0 11.7 9.4 ZnO 12.7 8.6 4.3 30.1 21.629.2 27.2 MgO 12.4 3.0 6.0 4.0 Li₂O 9.2 5.8 6.1 6.4 4.4 5.2 2.3 1.4 K₂O12.9 14.6 14.6 14.6 10.8 12.9 10.7 8.6 Na₂O TiO₂ 2.7 2.7 2.7 2.7 ZrO₂P₂O₅ ° C.-hr 750-2 600-1 600-1 600-1 650-1 650-1 650-1 650-1Glass-Ceramic Transparency 2 4 4 3 2 2 2 3 25 26 27 28 29 30 31 32 SiO₂56.0 58.3 61.3 61.5 60.5 63.0 64.5 63.2 Al₂O₃ 1.6 12.8 7.8 7.7 8.0 8.28.0 ZnO 30.2 26.6 13.9 18.6 20.9 19.0 12.7 13.4 MgO 2.9 Li₂O 10.3 9.57.9 8.0 6.9 9.2 10.9 8.2 K₂O Na₂O TiO₂ ZrO₂ P₂O₅ 3.5 4.0 4.1 4.1 4.0 3.82.8 4.3 ° C.-hr 575-2 575-2 650-2 700-2 600-2 600-2 600-4 600-2 650-4700-4 700-4 750-4 750-4 750-4 650-4 Glass-Ceramic Transparency 1 2 3 2 44 4 3

Table 8 (U.S. Pat. No. 6,632,757):

TABLE 8 1 2 3 4 5 6 7 8 9 SiO₂ 50.4 50.1 50.4 51.5 37.6 51.3 48.7 48.145.6 GeO₂ — — — — 16.2 — — — — Al₂O₃ 14.7 14.0 14.7 15.0 13.7 16.4 17.714.0 18.7 MgO 21.4 22.9 21.4 21.9 19.9 17.2 17.2 25.0 18.4 K₂O 13.5 13.013.5 10.5 12.6 15.1 16.4 12.9 17.3 Li₂O — — — 1.0 — — — — — TiO₂| — —4.0 — 3.7 5.0 5.0 5.0 4.9 Cr₂O₃*| 0.1 — 0.1 — 0.07 0.25 0.155 0.1550.155 As₂O₅| 0.4 — — — — — — — — Glass green clear olive olive olivedark green, green, green, green green green green olive olive olive tinttint tint Ceram. 750-4 750-4 750-2 750-2 750-2 750-2 750-8 750-4 750-4Cycle 850-2 900-2 900-4 850-4 900-4 900-4 900-2 900-1 900-1 ° C.-hrGlass- medium medium fine medium fine very fine very fine very fine veryfine Ceramic grained; grained; grained; grained grained grained;grained; grained; grained; translucent translucent translucenttransparent translucent translucent translucent translucent brownishbrown green- green- green- brown brown brown Crystal forsteriteforsterite forsterite forsterite forsterite forsterite forsteriteforsterite forsterite Phase |excess of 100% *total chromium oxide asCr₂O₃

Table 9 (U.S. Pat. No. 6,632,758):

TABLE 9 1 2 3 4 5 6 7 8 SiO₂ 38.7 41.5 41.5 39.6 39.9 38.6 37.0 33.1Ga₂O₃ 42.3 30.0 30.0 31.0 26.9 25.1 24.0 33.4 Al₂O₃ 7.7 16.3 16.3 15.918.9 16.2 15.5 14.0 Li₂O 1.2 1.3 1.3 2.0 1.8 — — — Na₂O 10.0 10.8 10.8 —— — — — K₂O 11.5 12.5 15.1 14.5 12.9 La₂O₃ 4.0 8.0 5.3 MgO 1.0 1.0 1.3TiO₂ NiO* 0.5 0.5 0.5 0.1 0.1 0.1 0.1 Cr₂O₃* 0.5 As₂O₅ 0.6 Glass Colorred- red- olive- red- red- red- red- light brown brown green brown brownbrown brown blue Heat 850°-2 775°-2 725°-2 750°-8 750°-8 850°-2 900°-2900°-2 Treatment 775°-2 900°-2 900°-2 (° C./hr.) Glass- blue- blue-yellow- blue- blue- blue- blue blue ceramic green green olive greengreen green Color Degree of high medium medium high high high mediummedium Transparency Crystal Phase Alumino- Alumino- Alumino- Alumino-Alumino- Alumino- Alumino- Alumino- gallate gallate gallate gallategallate gallate gallate gallate spinel spinel spinel spinel spinelspinel spinel spinel 9 10 11 12 13 14 15 16 SiO₂ 27.5 38.6 37.3 43.539.4 37.5 35.7 41.3 Ga₂O₃ 42.3 25.1 30.6 28.5 30.2 31.0 29.5 22.8 Al₂O₃11.7 16.2 13.3 14.3 16.4 15.9 15.1 21.8 Li₂O — — — — 1.0 1.0 1.0 2.1Na₂O — — — — — — — — K₂O 10.8 15.1 12.3 13.2 13.0 14.7 14.0 12.0 11.5La₂O₃ 6.7 4.0 5.3 — — — — — MgO 1.0 1.0 1.1 0.5 — — — — TiO₂ — — — — — —4.8 — NiO* 0.5 — 0.1 0.1 0.1 — — 0.5 Cr₂O₃* — 0.8 — — — — — — As₂O₅ — —— — — 0.05 0.05 — Glass Color blue- green light light blue- color-color- red- green blue blue green less less brown Heat 850°-2 850°-2900°-2 900°-2 700°-2 750°-4 750°-4 750°-8 Treatment 900°-2 900°-2 900°-2900°-2 (° C./hr.) Glass- blue- blue- yellow- blue- blue- color- color-blue ceramic green green olive green green less less Color Degree of lowmedium medium medium medium medium low high Transparency Crystal PhaseAlumino- Alumino- Alumino- Alumino- Alumino- Alumino- Alumino- Alumino-gallate gallate gallate gallate gallate gallate gallate gallate spinelspinel spinel spinel spinel spinel spinel spinel 20 21 22 23 24 25 SiO₂38.6 38.6 38.6 38.6 38.6 38.6 Ga₂O₃ 25.1 25.1 25.1 25.1 25.1 25.1 Al₂O₃16.2 16.2 16.2 16.2 16.2 16.2 K₂O 15.1 15.1 15.1 15.1 15.1 15.1 La₂O₃4.0 4.0 4.0 4.0 4.0 4.0 MgO 1.0 1.0 1.0 1.0 1.0 1.0 Co₂O₃* 0.005 0.010.03 0.08 0.12 0.20 Glass Color Blue Blue Blue Blue Heat Treatment 900-2900-2 900-2 900-2 900-2 (° C./hr.) Glass-ceramic Blue Blue Blue BlueBlue Color Degree of high high high high high Transparency *Transitionmetal oxide dopants in excess of 100%.

Table 10 (U.S. Pat. No. 6,660,669):

TABLE 10 Forsterite Glass-Ceramic Compositions EXAMPLE NO. (wt %) OXIDESEx. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 SiO₂ 47.6 47.4 48.8 47.951.1 44.1 47.9 46.2 Al₂O₃ 13.3 13.3 11.3 13.5 13.1 17.5 13.4 12.0 MgO21.6 21.4 22.1 21.8 18.7 21.4 21.7 24.8 Na₂O 8.2 5.4 7.0 4.1 7.9 10.64.1 7.3 K₂O — 4.1 — 6.2 — — 6.2 — TiO₂ 9.1 8.2 10.7 6.4 9.1 6.2 6.3 9.0CrO₃ 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.6 Glass Quality Green, clear Green,clear Green, clear Green, clear Green, clear Green, clear Dark Green,Very clear Dark Green Heat Treatment 750 @ 8 750 @ 8 700 @ 8 700 @ 8 750@ 8 750 @ 8 700 @ 8 700 @ 8 Temp. (° C.) @ hrs. 850 @ 2 850 @ 2 850 @ 2850 @ 4 850 @ 2 900 @ 2 850 @ 4 850 @ 4 Glass-Ceramic Brown Brown BrownGreenish Olive Brown Greenish Dark Brown X-ray diffraction Forsterite,Forsterite, Forsterite, Forsterite Forsterite, Forsterite, ForsteriteForsterite, Crystal phase(s) Minor rutile Faint rutile rutile Minorenstatite Cordierite, Minor rutile Cristobolite Liquidus (° C.) 14501500 — 1500 — — 1500 —

Table 11 (U.S. Publ. No. 2012/0114955):

TABLE 11 SiO₂ 45-65 Al₂O₃ 14-28 ZnO  4-13 MgO 0-8 with ZnO + MgO ≥8 BaO0-8 SnO₂ 0.1-2  TiO₂ 2-4 ZrO₂  3-4.5 Fe₂O₃ <100 ppm. Examples A B C D 12 3 4 5 6 Composition (wt %) SiO₂ 59.00 59.30 60.28 60.04 60.25 59.4759.72 59.22 58.97 59.31 Al₂O₃ 19.00 19.10 19.41 19.33 19.40 19.13 19.2319.23 18.97 19.10 ZnO 8.95 9.00 9.15 9.11 9.15 9.08 9.06 9.06 9.00 9.07MgO 2.49 2.50 2.54 2.53 2.54 2.52 2.52 2.52 2.50 2.51 BaO 2.09 2.11 2.142.13 2.14 3.39 2.12 2.12 3.36 3.39 TiO₂ 4.97 5.00 3.43 3.42 2.35 2.302.33 2.33 2.28 3.40 ZrO₂ 2.98 2.99 3.05 3.04 3.95 3.87 3.92 3.92 3.843.02 SnO₂ 0.00 0.00 0.00 0.00 0.22 0.22 1.10 1.60 1.08 0.20 As₂O₃ 0.500.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SO₃ 0.4 Fe₂O₃ (ppm) 80 8080 44 44 44 44 44 44 44 (RO*/Al₂O₃) 0.99 0.99 0.99 0.99 0.99 1.05 0.990.99 1.05 0.93 Properties Transmission T at 400 nm % 82.8 72.8 76.7 82.781.6 83 65.8 57.8 70.9 78.6 T 80% (nm) 395 739 500 373 395 387 432 445426 407 T 85% (nm) 410 961 655 535 426 419 469 495 477 508 T 90% (nm)744 1350 1116 1099 726 697 756 837 801 960 Devitrification Liquidus1421-1435° C. 1442-1460° C. 1430- 1449-1469° C. 1445- 1430- (1000-12501447° C. (1270-1620 1461° C. 1449° C. (940-1130 dPa · s) Zircon dPa · s)Zircon (970- Zircon Zircon 1240 dPa · s) dPa · s) Mullite Mullite Strainpoint 925° C. 955° C. 935° C. 894° C. 890° C. CTE (10⁻⁷ K⁻¹) 36 *RO =MgO + BaO + ZnO (mol %)

Additional aspects that influence glass ceramic suitability as a coverglass include, but not limited to, radio and microwave frequencytransparency, 4-point bend strength, stiffness/Young's Modulus,hardness, crack indentation threshold, thermal conductivity, depth ofcompressive layer (DOL), surface compressive stress and central tension.Formability, finishing, design flexibility, and manufacturing costsassociated with this glass ceramic material also factor into whether theparticular glass ceramic material is suitable for use as the electronicdevice cover glass. Furthermore, the material selected may also dependon aesthetics including color, surface finish, weight, etc.

In one embodiment, the invention comprises a glass ceramic for use as anelectronic device cover glass comprising an ion exchanged glass ceramicmaterial exhibiting optical transparency in the visible spectrum, afracture toughness of greater than 0.6 MPa·M^(1/2), a 4-point bendstrength of greater than 350 MPa, a Vickers hardness of at least 600kgf/mm² and a Vickers median/radial crack initiation threshold of atleast 5 kgf, a Young's Modulus ranging between about 50 to 100 GPa, athermal conductivity of less than 2.0 W/m° C. In another embodiment, theglass ceramic exhibits a fracture toughness of greater than 1.0MPa·m^(1/2), an MOR of greater than 135 MPa, a Knoop hardness of atleast 400 kg/mm², a thermal conductivity of less than 4 W/m° C. and aporosity of less than 0.1%. In a still further embodiment, the glassceramic cover glass comprises a glass-ceramic which exhibits a fracturetoughness of greater than 1.0 MPa·m^(1/2), an MOR of greater than 275MPa, preferably greater than 350 MPa. In a still further embodiment theelectronic cover glass comprises a glass ceramic which exhibits afracture toughness of greater than 0.70 MPa·m^(1/2), and an 4-point bendstrength of greater than 475 MPa, preferably greater than 525 MPa and aYoung's Modulus/elasticity ranging between 50 and 75 GPa.

Fracture toughness in a preferred embodiment can be as high as 1.2MPa·m^(1/2). In some embodiments, the fracture toughness may be as highas 5.0 MPa·m^(1/2).

In another aspect of the invention the glass-ceramic cover glass can besubject to an ion exchange process. At least one surface of theglass-ceramic article is subject to an ion exchange process, such thatthe one ion exchanged (“IX”) surface exhibits a compressive layer havinga depth of layer (DOL) greater than or equal to 2% of the overallarticle thickness and exhibiting a compressive strength of at least 300MPa.

Any ion exchange process known to those in the art is suitable so longas the above DOL and compressive strength are achieved. See, e.g., U.S.Pat. No. 5,127,931, herein incorporated by reference. Such a processwould include, but is not limited to submerging the glass ceramicarticle in a bath of molten Nitrate, Sulfate, and/or Chloride salts ofLithium, Sodium, Potassium and/or Cesium, or any mixture thereof. Thebath and samples are held at a constant temperature above the meltingtemperature of the salt and below its decomposition temperature,typically between 350 and 800° C. The time required for ion-exchange oftypical glass ceramics can range between 15 minutes and 48 hours,depending upon the diffusivity of the crystalline and glassy phases. Incertain cases, more than one ion-exchange process may be used togenerate a specific stress profile or surface compressive stress for agiven glass ceramic material.

Additionally, in cases where the glass ceramic is ion exchanged, theglass ceramic cover glass may exhibit at least one of the followingattributes: (i) a compressive surface layer having a depth of layer(DOL) greater than 20 μm and a compressive stress greater than 400 MPa,or, (ii) a central tension of more than 20 MPa. In an another exemplaryembodiment the glass ceramic cover glass exhibits an overall thicknessof 2 mm and compressive layer exhibiting a DOL of 40 μm with thatcompressive layer exhibiting a compressive stress of at least 525 MPa.Again, any ion exchange process which achieves these features issuitable.

In particular, the central tension CT within a glass ceramic article canbe calculated from the compressive stress CS. Compressive stress CS ismeasured near the surface (i.e., within 100 μm), giving a maximum CSvalue and a measured depth of the compressive stress layer (alsoreferred to herein as “depth of layer” or “DOL”). The relationshipbetween CS and CT is given by the expression:CT=(CS·DOL)/(t−2 DOL)  (1),wherein t is the thickness of the glass ceramic article. Unlessotherwise specified, central tension CT and compressive stress CS areexpressed herein in megaPascals (MPa), whereas thickness t and depth oflayer DOL are expressed in millimeters.

It should be noted that in addition to single step ion exchangeprocesses, multiple ion exchange procedures can be utilized to create adesigned ion exchanged profile for enhanced performance. That is, astress profile created to a selected depth by using ion exchange bathsof differing concentration of ions or by using multiple baths usingdifferent ion species having different ionic radii. Additionally one ormore heat treatments can be utilized before or after ion exchange totailor the stress profile.

This requisite fracture toughness in excess of 0.6 MPa·m^(1/2), incombination with 20 μm/surface compressive stress exceeding 400 MPacombination (or CT exceeding 20 MPa), the Vickers hardness/indentationthreshold requirements, and the 4-point bend strength of greater than350 MPa, all function to result in an cover glass which is sufficientlystrong and durable so as to withstand typical consumer use/applications.One measure of this durability feature which the aforementionedion-exchanged glass ceramic article is capable of meeting is the abilityof the ion exchanged glass ceramic article to withstand a standard droptesting requirement involving numerous (e.g., 5) impacts/drops from aheight of one meter onto a hard surface such as concrete or granite.

Referring now particularly to the thermal conductivity attribute, itshould be noted that thermal conductivities of the desired level,particularly of less than 4 W/m° C., are likely to result in a coverglass that remains cool to the touch even in high temperaturesapproaching as high as 100° C. Preferably, a thermal conductivity ofless than 3 W/m° C., and less than 2 W/m° C. are desired. Representativethermal conductivities* (in W/m° C.) for some suitable silicateglass-ceramics (discussed in detail below) include the following:

Cordierite glass-ceramic 3.6 β-spodumene (Corningware) 2.2 β-quartz(Zerodur) 1.6 Wollastonite (Example 9 - below) 1.4 Machinable mica(Macor) 1.3 *(see A. McHale, Engineering properties of glass-ceramics,in Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, ASMInternational 1991, hereby incorporated by reference in its entirety.)Other glass-ceramics which exhibit the requisite thermal conductivityfeature included lithium disilicate based and canasite glass ceramicsboth of which are expected to exhibit thermal conductivity value of lessthan 4.0 W/m° C. For comparison, it should be noted that a ceramic suchas alumina may exhibit undesirable thermal conductivities as high as 29.

In another exemplary embodiment the article, particularly the glassceramic cover glass exhibits radio and microwave frequency transparency,as defined by a loss tangent of less than 0.015 over the frequency rangeof between 500 MHz to 3.0 GHz. This radio and microwave frequencytransparency feature is especially important for wireless hand helddevices that include antennas internal to the device. This radio andmicrowave transparency allows the wireless signals to pass through thecover glass and in some cases enhances these transmissions. Furthermore,it may also be desirable to be transparent in the infrared to allowwireless optical communication between electronic devices; specificallyan infra-red transparency of greater than 80% at wavelengths rangingfrom 750 to 2000 nm. For example IR communication can be used todownload music files to a portable music player, or workout data can beuploaded from a GPS or heart rate monitor to a computer for analysis.

In certain embodiments the glass ceramic cover glass has at least onesurface exhibiting a Ra roughness of less than 50 nm, preferably lessthan 15 nm. In order to achieve this level of surface roughness, oneoption is to polish the surface using standard polishing techniques soas to achieve the requisite surface roughness of less than 50 nm,preferably less than 15 nm. Alternatively, the glass ceramic article canformed using a mold having a polished or non-textured surface so as toachieve the requisite surface roughness of less than 50 nm, preferablyless than 15 nm.

One specific glass ceramic is the β-quartz solid solution shown in Table12:

TABLE 12 SiO₂ 65.3 (wt %)  Al₂O₃ 20.1 (wt %)  B₂O₃ 2.0 (wt %) Li₂O 3.6(wt %) Na₂O 0.3 (wt %) K₂O MgO 1.8 (wt %) MgF₂ CaO CaF₂ SrO BaO ZnO 2.2P₂O₅ TiO₂ 4.4 ZrO₂ SnO₂ 0.3 Crystal (1) Strain (° C.) 792 Anneal (° C.)876 CTE (×10−7/° C.) 25-300° C. Density (g/cm³) 2.525 Liq. Temp 1210Liq. Visc (Poise) 18000 RoR Strength (MPa) 350 IX RoR Strength (MPa) 700Fract Tough (MPa 1 m^(1/2)) Modulus (Mpsi) 12.448 Shear Mod (Mpsi) 5.001P Ratio 0.245

Generally, the process for forming any of the representativeglass-ceramic materials detailed herein comprises melting a batch for aglass consisting essentially, in weight percent on the oxide basis ascalculated from the batch, of a composition within the range set forthabove. It is within the level of skill for those skilled in theglass-ceramic art to select the required raw materials necessary as toachieve the desired composition. Once the batch materials aresufficiently mixed and melted, the process involves cooling the melt atleast below the transformation range thereof and shaping a glass articletherefrom, and thereafter heat treating this glass article attemperatures between about 650-1,200° C. for a sufficient length of timeto obtain the desired crystallization in situ. The transformation rangehas been defined as that range of temperatures over which a liquid meltis deemed to have been transformed into an amorphous solid, commonlybeing considered as being between the strain point and the annealingpoint of the glass.

The glass batch selected for treatment may comprise essentially anyconstituents, whether oxides or other compounds, which upon melting toform a glass will produce a composition within the aforementioned range.Fluorine may he incorporated into the batch using any of the well-knownfluoride compounds employed for the purpose in the prior art which arecompatible with the compositions herein describe

Heat treatments which are suitable for transforming the glasses of theinvention into predominantly crystalline glass-ceramics generallycomprise the initial step of heating the glass article to a temperaturewithin the nucleating range of about 600-850° C. and maintaining it inthat range for a time sufficient to form numerous crystal nucleithroughout the glass. This usually requires between about ¼ and 10hours. Subsequently, the article is heated to a temperature in thecrystallization range of from about 800-1,200° C. and maintained in thatrange for a time sufficient to obtain the desired degree ofcrystallization, this time usually ranging from about 1 to 100 hours.Inasmuch as nucleation and crystallization in situ are processes whichare both time and temperature dependent, it will readily be understoodthat at temperatures approaching the hotter extreme of thecrystallization and nucleation ranges, brief dwell periods only will benecessitated, whereas at temperatures in the cooler extremes of theseranges, long dwell periods will be required to obtain maximum nucleationand/or crystallization.

Additionally, the heat treatment can be optimized to produce glassceramics with high transmission properties. Such procedures aredescribed in U.S. Publ. No. 2007/0270299, herein incorporated byreference in its entirety.

It will be appreciated that numerous modifications in thecrystallization process are possible. For example, when the originalbatch melt is quenched below the transformation range thereof and shapedinto a glass article, this article may subsequently be cooled to roomtemperature to permit visual inspection of the glass prior to initiatingheat treatment. It may also be annealed at temperatures between about550-650° C. if desired. However, where speed in production and fueleconomies are sought, the batch melt can simply be cooled to a glassarticle at some temperature just below the transformation range and thecrystallization treatment begun immediately thereafter.

Glass-ceramics may also be prepared by crystallizing glass fits in whatis referred to as powder processing methods. A glass is reduced to apowder state, typically mixed with a binder, formed to a desired shape,and fired and crystallized to a glass-ceramic state. In this process,the relict surfaces of the glass grains serve as nucleating sites forthe crystal phases. The glass composition, particle size, and processingconditions are chosen such that the glass under-goes viscous sinteringto maximum density just before the crystallization process is completed.Shape forming methods may include but are not limited to extrusion,pressing, and slip casting.

Various modifications and variations can be made to the materials,methods, and articles described herein. Other aspects of the materials,methods, and articles described herein will be apparent fromconsideration of the specification and practice of the materials,methods, and articles disclosed herein. It is intended that thespecification and examples be considered as exemplary.

We claim:
 1. An article suitable as a cover glass for a portableelectronic device, the article comprising a glass ceramic, the glassceramic having a primary crystalline phase of transparent spinel ortransparent mullite and exhibiting: a. optical transparency of greaterthan 60%, as defined by the transmission of light over the range of from400-750 nm through 1 mm of the glass ceramic; b. colorlessness, asdefined by having the values of L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4 onthe CIE 1976 Lab color space as measured in transmission through 1 mm ofglass ceramic; c. at least one of the following attributes: (i) afracture toughness of greater than 0.60 MPa·m^(1/2;) (ii) a 4-point bendstrength of greater than 350 MPa; (iii) a Vickers hardness of at least450 kgf/mm²; (iv) a Vickers median/radial crack initiation threshold ofat least 5 kgf; (v) a Young's Modulus ranging between 50 to 100 GPa; and(vi) a thermal conductivity of less than 2.0 W/m° C.; and d. at leastone of the following attributes: (i) a compressive surface layer havinga depth of layer (DOL) greater than or equal to 20 μm and a compressivestress greater than 400 MPa, or, (ii) a central tension of more than 20MPa.
 2. The article of claim 1 wherein the glass ceramic exhibitsoptical transparency of greater than 80%, as defined by the transmissionof light over the range of from 400-750 nm through 1 mm of the glassceramic.
 3. The article of claim 1, wherein the glass ceramic exhibitscolorlessness, as defined by having color space coordinates of L*≥90;0.08≥a*≥−0.08; and 0.3≥b*≥−0.3 on the CIE 1976 Lab color space asmeasured in transmission through 1 mm of glass ceramic.
 4. The articleof claim 1, wherein the glass ceramic is ion exchanged.
 5. The articleof claim 4, wherein the glass ceramic exhibits an overall thickness of1.2 mm and compressive layer exhibiting a DOL of ranging between 40 to80 μm and the compressive layer exhibits a compressive stress of 525MPa.
 6. The article claimed in claim 1 wherein the glass ceramicexhibits a Young's Modulus ranging between 50 and 75 GPa.
 7. The articleclaimed in claim 1 wherein the glass ceramic exhibits an 4-point bendstrength of greater than 475 MPa.
 8. The article claimed in claim 1wherein the glass ceramic exhibits a Vickers hardness of at least 500kgf/mm² and Vickers median/radial crack initiation threshold of greaterthan 10 kgf.
 9. The article of claim 1, wherein the glass ceramicexhibits a thermal conductivity of less than 1.5 W/m° C.
 10. The articleof claim 1, wherein the glass ceramic is transparent and exhibits atleast one surface having a Ra roughness of less than less than 50 nm.11. The article of claim 1, wherein the glass ceramic exhibits anear-infra-red transparency of greater than 80% at a wavelength rangingfrom 750 to 2000 nm.
 12. The article of claim 1, wherein the glassceramic is fusion formable.
 13. An article suitable as a cover glass fora portable electronic device, the article comprising a glass ceramic,the glass ceramic having a primary crystalline phase of transparentspinel or transparent mullite and exhibiting: a. optical transparency ofgreater than 60%, as defined by the transmission of light over the rangeof from 400-750 nm through 1 mm of the glass ceramic; b. colorlessness,as defined by having the values of L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4on the CIE 1976 Lab color space as measured in transmission through 1 mmof glass ceramic; and c. at least one of the following attributes: (i) afracture toughness of greater than 1.0 MPa·m^(1/2); (ii) an MOR ofgreater than 135 MPa (iii) a Knoop hardness of at least 400 kg/mm²; (iv)a thermal conductivity of less than 4 W/m° C.; and (v) a porosity ofless than 0.1%.
 14. An electronic device comprising the article of anyof claims 11 and
 12. 15. An article suitable as a cover glass for aportable electronic device, the article comprising a transparent glassceramic, the transparent glass ceramic having a primary crystallinephase selected from the group consisting of β-quartz, β-spodumene,spinel, mullite, forsterite, enstatite, lithium silicate, and fluormicaand exhibiting: a. optical transparency of greater than 80%, as definedby the transmission of light over the range of from 400-750 nm through 1mm of the glass ceramic; b. colorlessness, as defined by having thevalues of L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4 on the CIE 1976 Lab colorspace as measured in transmission through 1 mm of glass ceramic; and c.at least one of the following attributes: (i) a fracture toughness ofgreater than 0.60 MPa·m^(1/2); (ii) a 4-point bend strength of greaterthan 350 MPa; (iii) a Vickers hardness of at least 450 kgf/mm²; (iv) aVickers median/radial crack initiation threshold of at least 5 kgf; (v)a Young's Modulus ranging between 50 to 100 GPa; or (vi) a thermalconductivity of less than 2.0 W/m° C.
 16. The article of claim 15,wherein the glass ceramic exhibits colorlessness, as defined by havingcolor space coordinates of L*≥90; 0.08≥a*≥−0.08; and 0.3≥b*≥−0.3 on theCIE 1976 Lab color space as measured in transmission through 1 mm ofglass ceramic.
 17. The article of claim 15, wherein the glass ceramic ision exchanged.
 18. The article of claim 17, wherein the glass ceramicexhibits an overall thickness of 1.2 mm and a compressive layerexhibiting a DOL ranging between 40 to 80 μm and the compressive layerexhibits a compressive stress of 525 MPa.
 19. The article of claim 15,wherein the glass ceramic exhibits a Young's Modulus ranging between 50and 75 GPa.
 20. The article of claim 15, wherein the glass ceramicexhibits an 4-point bend strength of greater than 475 MPa.
 21. Thearticle of claim 15, wherein the glass ceramic exhibits a Vickershardness of at least 500 kgf/mm² and Vickers median/radial crackinitiation threshold of greater than 10 kgf.
 22. The article of claim15, wherein the glass ceramic exhibits a thermal conductivity of lessthan 1.5 W/m° C.
 23. The article of claim 15, wherein the glass ceramicis transparent and exhibits at least one surface having a Ra roughnessof less than less than 50 nm.
 24. The article of claim 15, wherein theglass ceramic exhibits a near-infra-red transparency of greater than 80%at a wavelength ranging from 750 to 2000 nm.
 25. The article of claim15, wherein the glass ceramic is fusion formable.
 26. The article ofclaim 15, wherein the glass ceramic exhibits a fracture toughness ofgreater than 0.60 MPa·m^(1/2).
 27. The article of claim 15, wherein theglass ceramic exhibits a 4-point bend strength of greater than 350 MPa.28. The article of claim 15, wherein the glass ceramic exhibits aVickers hardness of at least 450 kgf/mm².
 29. The article of claim 15,wherein the glass ceramic exhibits a Vickers median/radial crackinitiation threshold of at least 5 kgf.
 30. The article of claim 15,wherein the glass ceramic exhibits a Young's Modulus ranging between 50to 100 GPa.
 31. The article of claim 15, wherein the glass ceramicexhibits a thermal conductivity of less than 2.0 W/m° C.
 32. An articlesuitable as a cover glass for a portable electronic device, the articlecomprising a transparent glass ceramic, the transparent glass ceramichaving a primary crystalline phase selected from the group consisting ofβ-quartz, β-spodumene, spinel, mullite, forsterite, enstatite, lithiumsilicate, and fluormica thereof and exhibiting: a. optical transparencyof greater than 80%, as defined by the transmission of light over therange of from 400-750 nm through 1 mm of the glass ceramic; b.colorlessness, as defined by having the values of L*≥90; 0.1≥a*≥−0.1;and 0.4≥b*≥−0.4 on the CIE 1976 Lab color space as measured intransmission through 1 mm of glass ceramic; and c. at least one of thefollowing attributes: (i) a fracture toughness of greater than 1.0MPa·m^(1/2); (ii) an MOR of greater than 135 MPa (iii) a Knoop hardnessof at least 400 kg/mm²; (iv) a thermal conductivity of less than 4 W/m°C.; or (v) a porosity of less than 0.1%.
 33. The article of claim 32,wherein the glass ceramic exhibits a fracture toughness of greater than1.0 MPa·m^(1/2).
 34. The article of claim 32, wherein the glass ceramicexhibits an MOR of greater than 135 MPa.
 35. The article of claim 32,wherein the glass ceramic exhibits a Knoop hardness of at least 400kg/mm².
 36. The article of claim 32, wherein the glass ceramic exhibitsa thermal conductivity of less than 4 W/m° C.
 37. The article of claim32, wherein the glass ceramic exhibits a porosity of less than 0.1%. 38.An electronic device comprising the article of claim 15 or claim 32.